The invention is particularly applicable to satellite signal transmission in which, in particular, the Doppler effect, satellite drift, and the instability of the frequency transposition equipment impart frequency shifts to the received signal. At a ground station receiving the signal from the satellite, a rotation of the constellation is observed after the received signal has been transposed into baseband, and this does not enable the received signal to be sampled at the instants when the eye diagram is at its most open.
FIG. 1 is a block diagram of a digital signal receiver including an estimator for estimating the frequency difference existing between the carrier frequency of a received digital signal and the frequency of a receive local oscillator.
The receiver shown in FIG. 1 includes a direct-conversion quadrature demodulator to which the received signal, referenced SR, is applied. More precisely, the signal SR is applied to two mixers 10, 11 also receiving respective demodulation signals from a local oscillator 12, one of the mixers (10) receiving the demodulation signal via a 90.degree. phase-shifter referenced 13. The mixers 10 and 11 deliver baseband signals to respective analog-to-digital converters (ADC) 14, 15 followed by receive filters 16, 17 that are conventionally Nyquist root band-pass pass filters (complex Nyquist filters). The filters 16 and 17 reject adjacent channels outside the band of the modulated signal SR and deliver respective trains I and Q respectively representing the in-phase component and the quadrature component of the modulated signal SR. Each received symbol is encoded over K samples (for example, K is equal to 4). The trains I and Q are applied to an estimator 18 serving to determine the frequency difference between the signal SR and the output signal of the local oscillator 12. Said frequency difference is referenced .epsilon. and is applied to the local oscillator 12 so as to perform automatic frequency control.
The estimator 18 operates conventionally using the Alberty algorithm. The drawback with that algorithm is that it does not work when interference is present for frequency shifts greater than Ds/4, where Ds is the symbol rate. In satellite transmission, such interference is constituted by the channels adjacent to the demodulated channel. Whenever one of the adjacent channels enters the band of a receive filter at .+-.Ds/2, the estimator 18 locks onto the adjacent channel and the frequency difference can no longer be measured. As a result the expected signal is lost.